The present invention relates to a piezoelectric element. More particularly, the present invention relates to a piezoelectric element which has very high piezoelectric properties, which is superior in vibration transmittability between ceramic substrate and piezoelectric, and which can provide an actuator or sensor of small size and high integration; as well as to a process for producing such a piezoelectric element.
In recent years, piezoelectric elements have been used in ink jet printer heads, speakers, microphones, etc. As the piezoelectric elements, there are known those comprising a ceramic substrate, a piezoelectric made of a ceramic composition, formed on the substrate, and electrodes electrically connected to the piezoelectric. As to the ceramic composition constituting the piezoelectric, various improved compositions have been disclosed.
For example, a PbTiO3xe2x80x94PbZrO3 binary system solid solution composition, a Pb(Mg1/3Nb2/3)O3xe2x80x94PbTiO3xe2x80x94PbZrO3 ternary system solid solution composition and a composition wherein part of Pb and Mg in either of the above composition is replaced by Ni, Nb, Mn, etc., were disclosed [JP-B-50-3519, JP-B-60-102779, Journal of The American Ceramic Society; 49[11] 577 (1966)]. These compositions are improved in piezoelectric properties (e.g. piezoelectric d constant) which are the most important factor determining the piezoelectric element properties. By using a piezoelectric made of such an improved ceramic composition, a piezoelectric element having superior piezoelectric properties is expectable.
When a piezoelectric element is produced actually by coating a piezoelectric material made of the above ceramic composition, on a ceramic substrate and then heat-treating the coated piezoelectric material, however, the obtained piezoelectric has a low density; therefore, there have been pointed out such problems that the piezoelectric element has a low flexural displacement or, when a voltage is applied thereto, it causes dielectric breakdown at the low density area.
Hence, there has been used a piezoelectric element produced by heat-treating beforehand a piezoelectric material made of the above ceramic composition and then attaching this piezoelectric onto a ceramic substrate (JP-A-11-29357).
In this piezoelectric element, attention was paid to a fact that use of a ceramic substrate impairs the densification of a piezoelectric formed on the ceramic substrate. Thus, the piezoelectric element was accordingly improved in piezoelectric properties by means of obtaining a dense piezoelectric by heat-treating a piezoelectric material made of a ceramic composition before attachment to a substrate.
In this piezoelectric element, however, it is necessary to use an inorganic or organic adhesive at the time of attaching the piezoelectric onto the ceramic substrate. Therefore, there have been such problems that the adhesive impairs the vibration transmittability between the ceramic substrate and the piezoelectric or the adhesive components infiltrate into the piezoelectric or the ceramic substrate, deteriorating their properties.
Further, the above-mentioned conventional ceramic compositions have had no sufficiently satisfactory piezoelectric properties.
The present invention has been completed in view of the above-mentioned problems and aims at providing a piezoelectric element which has very high piezoelectric properties, which is superior in vibration transmittability between ceramic substrate and piezoelectric, and which can provide an actuator or sensor of small size and high integration, and a process for producing such a piezoelectric element.
The present inventor made a study in order to solve the above-mentioned problems. As a result, the present inventor found out that when there is used a piezoelectric material composed mainly of a PbMg1/3Nb2/3O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition of particular formulation and containing NiO in a particular proportion, the piezoelectric material can be densified even when it is coated on a ceramic substrate and then heat-treated, whereby can be obtained a piezoelectric having very high piezoelectric properties. The present invention has been completed based on the above finding.
According to the present invention, there is provided a piezoelectric element including a ceramic substrate, and a piezoelectric(s) made of a ceramic composition composed mainly of a PbMg1/3Nb2/3O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition, the chemical formula of which is discussed below. The piezoelectric(s) contain 0.05 to 10.0% by weight, based on the ceramic composition, of NiO. Electrodes are electrically connected to the piezoelectric, which is solidly attached to the ceramic substrate directly or via part or all of the electrodes.
The piezoelectric ceramic composition is represented by the following general formula:
Pbx(Mgy/3Nb2/3)aTibZrcO3,
wherein 0.95xe2x89xa6xxe2x89xa61.05; 0.8xe2x89xa6yxe2x89xa61.0; a, b and c are decimals falling in a range surrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325, 0.125), (0.375, 0.325, 0.300), (0.100,0.425,0.475), (0.100,0.475,0.425) and (0.375,0.425,0.200), and a+b+c=1.000.
In the piezoelectric element of the present invention, it is preferred that NiO is dispersed in the ceramic composition in such a concentration gradient that the concentration of NiO becomes higher from the interface between the piezoelectric and the ceramic substrate or the electrode(s) towards the thickness direction of the piezoelectric.
Pb in the ceramic composition may be replaced by at least one kind of element selected from the group consisting of Sr, Ca and Ba, in an amount of 2 to 10 mole %. Pb in the ceramic composition may also be replaced by La in an amount of 0.2 to 1.0 mole %.
The ceramic substrate preferably has a thickness of 3 xcexcm to 1 mm, and the piezoelectric preferably has a thickness of 1 to 300 xcexcm. The ratio of the thickness of the ceramic substrate to the thickness of the piezoelectric (the thickness of the ceramic substrate/the thickness of the piezoelectric) is preferably 0.1 to 30.
According to the present invention, there is also provided a process for producing a piezoelectric element, which includes the steps:
coating, on a ceramic substrate or on an electrode(s) formed on a ceramic substrate, a piezoelectric material made of a ceramic composition composed mainly of a PbMg1/3Nb2/3O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition, the chemical formula of which is discussed below. The piezoelectric material contains 0.05 to 10.0% by weight, based on the ceramic composition, of NiO; and
heat-treating the coated piezoelectric material in the co-presence of an atmosphere-controlling material having a formulation of the ceramic NiO content as the ceramic composition or a formulation of higher NiO content than the ceramic composition.
The piezoelectric ceramic composition is represented by the following general formula:
Pbx(Mgy/3Nb2/3)aTibZrcO3,
wherein 0.95xe2x89xa6x 1.05; 0.8xe2x89xa6yxe2x89xa61.0; a, b and c are decimals falling in a range surrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325, 0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475, 0.425) and (0.375, 0.425, 0.200), and a+b+c=1.000.
According to the present invention, there is also provided a process for producing a piezoelectric element, which includes the steps of:
preparing a plurality of different piezoelectric materials each made of a ceramic composition composed mainly of a PbMg1/3Nb2/3O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition, the chemical formula of which is discussed below, and having a different NiO content;
coating in order of the low NiO content these piezoelectric materials on a ceramic substrate or on an electrode(s) formed on a ceramic substrate; and
heat-treating the coated piezoelectric materials.
The piezoelectric ceramic composition is represented by the following general formula:
Pbx(Mgy/3Nb2/3)aTibZrcO3,
wherein 0.95xe2x89xa6xxe2x89xa61.05; 0.8xe2x89xa6yxe2x89xa61.0; a, b and c are decimals falling in a range surrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325, 0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475, 0.425) and (0.375, 0.425, 0.200), and a+b+c=1.000.
The piezoelectric element according to the present invention can be used in capacitors or various sensors, as a dense and small dielectric element or pyroelectric element.